The invention relates to a device for facilitating the testing of tensile properties of materials and, in particular, to an apparatus which can be used to test the tensile characteristics of a materials utilizing a compressive load.
The accuracy of tensile test results are highly dependent upon reduction or elimination of secondary stresses induced in the specimen during the test procedure. The reduction or elimination of these secondary stresses provides test results which are more easily analyzed as failure is less likely to be attributable to non-tensile failure factors. In testing the tensile strength of brittle materials, such as rock, the reduction of secondary stresses is critical. The application of tensile loads to rock introduces difficulties in gripping the specimens without damaging the surface, in applying the load concentric with the axis of the specimen to prevent bending of the sample, and in avoiding abnormal stress concentrations at the grip ends.
Various methods for tensile testing of rock and other brittle materials have come into practice. Some involve the use of universal joints, flexible cables or similar mechanisms to obtain concentric load transfer while others involve the use of different bonding media at the gripping ends to eliminate stress concentrations associated with clamping. These direct methods rely on either very elaborate and time-consuming sample preparation or the use of highly specialized equipment. Such requirements have led to the development of indirect methods of approximating tensile strength. Of the known indirect methods, the Brazilian disc test is one of the most commonly used in the field of rock mechanics due to the ease of sample preparation and set-up. The sample consists of a disc of rock cut from a core and having a length to diameter ratio of 1:2. The specimens are loaded to failure at diametrically opposed surfaces using a compression press. The test is based on the experimental fact that most rocks in biaxial stress fields fail in tension at their tensile strength when one principal stress is tensile and the other finite principal stress is compressive with a magnitude not exceeding three times that of the tensile principal stress. Although there is some doubt as to the accuracy and even the validity of the Brazilian test, it continues to be used as a common tensile approximation, even though the direct-pull methods are considered to be most accurate. Unfortunately, laboratories equipped only with compression testing machines and/or having limited financial resources must rely on such indirect methods.